The Philips BX115U is one of the smallest Philips radio's with only MW
broadcast reception. It has no extra features at all. The dial scale is
simply made of white carton with the printed text on it. The technical
circuit is very straight forwarded and very easy. The schematic diagram can
be used as a reference to build a super hetero-dyne radio by yourself.
The Philips BX115U can be considered as the predecessor of the Philips B0X15U, but the Philips BX115U uses a
variable tuning capacitor.
Repairs:
The radio is repaired in the past. The coupling capacitor is renewed then
and on the UY41 there was a series network of a diode BY100 and a power
resistor parallel connected to the diode part of the UY41. This results
often in a too high DC voltage. Probably this was done to improve the
performance, but the dangerous part is, the tubes withdraw too much current
and worn then out earlier. The radio becomes then also more overheated
earlier. After removing the BY100 and the resistor, the radio plays well and
further measurements shows the high voltages are satisfying.
The reception sensitivity isn't well and the volume potentiometer cracks a
lot. After re-aligning the HF part and setting the dial needle at the proper
position, the reception of the broadcast stations is much more sensitive.
After one hour playing, the paper/tar capacitor C19 of 22 nF has exploded.
For optimal results and optimal safety the radio is furthermore repaired.
Paper/tar capacitor C19 of 22 nF is replaced by 22 nF 1000V.
Reason: Exploded.
Output transformer capacitor of 22 nF is replaced by 22 nF 1000V.
Reason: Increased value of the paper/tar capacitor which results into a
too dark and dull sound.
Coupling capacitor C17 of 10 nF 400V is renewed again by 10 nF 630V.
Reason: Extended life time.
Grid capacitor for the UBC41, C16 of 10 nF is replaced by 10 nF 630V.
Reason: Due to leakage, the anode voltage of the UBC41 was too low.
Decoupling capacitor C12 of 100 nF is replaced by 100 nF 1000V.
Reason: Due to leakage, the screen voltage for the UCH42 and UF41 is too
low. Now the reception sensitivity has improved.
C7 of 47 nF for the AGC is replaced by 47 nF 1000V.
Reason: The AGC is too slow due to increased capacitance. Due to leakage
the reception of strong broadcast stations, the sound has also
distortion.
C5 of 1000 pF is replaced by 1000 pF 2000V.
Reason: Optimal safety and low touch current at the antenna input.
After replacing these components, the radio meets now the factory
specifications, but after an hour playing, the audio amplifier start to
oscillate. Some modifications are also done to improve the audio quality
and improving the stability of the audio amplifier.
Radio start to oscillate, which results in a whistling sound.
Solution: reversed the primary wires of the output transformer.
Tube socket of the UL41 shows traces of overheating and is renew by a
ceramic type tube socket.
Cathode resistor UL41, R10 of 150R is replaced by 180R 3W.
Reason: Keeping the current and anode dissipation below their absolute
limits.
Added a 1K 1W stopper resistor to the grid of the UL41.
Reason: Improved stability of the power amplifier.
Added a 1K 3W resistor the the screen of the UL41.
Reason: Improved stability. A bit less current through the UL41.
After placing an AM detection network to the volume potentiometer,
preventing the negative voltage isn't sufficient. The carbon taper of
50+450K is defective and in fact the volume potentiometer need to be
replaced. Since these potentiometer has different mechanical
constructions, different axis and a switch, replacing the carbon taper is
sufficient.
Volume potentiometers cracks too much.
Solution: Carbon taper of 50+450K is replaced by 0M4+1M6.
Since a different carbon taper is used, the AM detection must also be
made to the specified requirements. Mostly the load of an AM detector is
between 250K to 500K. To improve the sound quality, a loudness circuit is
also added. See the small schematic diagram below.
The red parts are the added parts. R2 of 470K gives an optimal load for the
AM detector. C2 of 10 nF is the coupling capacitor and the the potentiometer
rid of the negative AM detection voltage. R1 and C1 is a loudness network,
which increases the bass and makes the sound more nice and full.
Performance:
After repairing this radio, the reception of the broadcast stations is very
well, even the weak broadcast stations are well.
The sound quality has improved a lot with the added loudness network.
The power amplifier has also improved a lot for long term stability and
reliability.
A new NOS potentiometer do also a very good job.
Disclaimer:
This
radio is directly supplied from the mains power.The high voltage supply is directly taken from the
mains power, which means the radio chassis carries the 230 Volt~ mains
voltage, which is very dangerous. Before making any repairs on the radio
or playing the radio without the back plate, use a safety transformer with
separate windings. The insulation of the windings must withstand at least
4000 Volts AC as well as a low leakage current of 0.5 mA~ or less.
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